The course of HIV-1 epidemic in Thailand has been decreased dramatically since its peak in the early 1990s. Although Thailand has had substantial success in HIV prevention efforts, close to 30,000 new infections continue to occur each year. Pattern of risk behavior may have been changed since the peak of the epidemic. Studies during the early phase of the epidemic found that more than 90% of HIV infected men reported having sex with commercial sex workers as the risk behavior while it was only 1% for IDU. HIV-1 prevalence in IDU population showed the stable high prevalence around 40-50% since 1989 including the latest round of the surveillance. Thailand has established HIV surveillance among about 60,000 21-year-old military conscripts annually. It is believed to be the nationally representative sample of young Thai men. This ongoing total survey is helpful to be studied for the changes of risk behavioral pattern in this population. HCV infection is prevalence in Thailand. To date the information about risk factors for HCV infection in Thailand was still limited. HCV antibody was believed to be a good surrogate marker for IDU in high HCV prevalence counties including Thailand. The long term goal of this study is to characterize the recent risk behavior for HIV-1 infection and the relationship between HIV-1 and HCV infections among young Thai men. Specific aims are:1. To characterize the pattern of high risk behaviors for HIV-1 infection among young Thai men aged 18-21 years old during 2005-2008 according to geographic area and the changes of the risk behaviors over time 2. To characterize prevalence of HCV infection according to geographic area and related risk factors among HIV-1 positive and negative young Thai men aged 18-21 years old and to assess the independent association between HIV-1 and HCV infections during 2005- 2008. We propose a case control study of HIV-1 infected young men and their 1:4 controls matched on year of birth, district of residence, and type of conscription.